Generally, a thermal recording material has a substrate and a heat-sensitive recording layer formed thereon, the heat-sensitive recording layer containing, as main components, a generally colorless or light-colored electron-donating dye precursor and an electron-accepting developer that reacts under heat to cause the dye precursor to develop a color. When the thermal recording material is heated with a thermal head, a hot pen or a laser beam, the dye precursor and the electron-accepting developer readily react with each other to give a recording image. Such thermal recording materials give recordings with a relatively simple apparatus and have advantages that their maintenance is easy and that they cause no noise. They are used in broad fields of measuring recorders, facsimile machines, printers, computer terminals, labels, automatic vending machines of railway tickets, and the like.
In recent years, particularly, thermal recording materials are used as accounting-related recording sheets such as gas, water and electricity bills, ATM slips of banking facilities and various receipts.
While the use and demands of thermal recording materials have been broadened in various ways, decreasing of residues adhering to a thermal head is required in addition to high thermal response and high color density that are basic properties.
That is, in the above thermal recording material for use as an accounting-related recording sheet, an unclear printing caused by residues adhering to a thermal head or the like during a long-distance printing cannot be permitted. This is true to a higher degree with regard to a portable printer using a sheet for the above bills, since it is difficult to have the portable printer checked for maintenance, for example, to have a thermal head cleaned, when a meter checker is outside.
Since it is difficult to attain an improvement in the above property on the basis of a thermal recording layer alone, JP-A-61-249789, JP-A-62-55189, etc., propose methods in which a protective layer for overcoming the adherence of residues to a thermal head and attaining improvements in other properties is formed on a thermal recording layer.
Under the circumstances, however, no thermal recording materials that cause fewer residues to adhere to a thermal head have been provided even if the above methods are employed, and it is still demanded to improve the above property on the basis of a thermal recording layer alone for decreasing energy and a cost.
For improving thermal response, a sensitizer is added as required. The sensitizer has the activity of promoting a color-forming reaction by dissolving or including a dye precursor and an electron-accepting developer near it when the sensitizer itself is melted by transmitted heat energy. It is therefore one of means of increasing the sensitivity of a thermal recording material to improve the compatibility thereof with the dye precursor and the electron-accepting developer.
As the above means, JP-A-48-19231 discloses embodiments in which waxes are added, JP-A-57-64593 discloses embodiments in which a naphthol derivative is added, and JP-A-60-56588 discloses embodiments in which diphenoxyethanes are added. Since, however, the addition of these sensitizers involves an increase in the amount of a heat-fusible substance, it is very difficult to decrease the amount of residues adhering to a thermal head.